Try to be as specific as possible. There is no such thing as too much detail here. Reproducing bugs can be tricky, so any info would be very much appreciated.
Also, add screenshots if possible. Screenshots can be taken using the F10 key ingame, and are stored in /data/screenshots

Locations can also be printed. In order to do that, type this into the chat window:
/location

Please use the following template when reporting bugs:



Thread Subject:

[Location] Bug summary

Message:
Location: 
Involved NPC(s): 
Quest Name:
Repro Rate: (Always, Random, Once, N.A.) 
Summary:

Example:

Thread Subject:
[Hawksmouth] Vednidher despawning after escort

Message:
Location: Hawksmouth Graveyard, 40000, -6273, 760
Involved NPC(s): Vednidher
Quest Name: A Token of Sincerity
Repro Rate: Always
Summary: After bringing Illas' Pendant to Vednidher in Hawksmouth, you have to lure him to the graveyard. Everytime I reached the graveyard with him, he disappeared after turning attackable. I tried it 3 times and varied the routes to the graveyard, it happened every time.